When a metal tube is bent, it becomes difficult to secure a desired shape precision due to spring-back after bending. Therefore, bending has generally been carried out factoring in an amount of spring-back. However, the amount of spring-back varies depending on the material, the size of the tube, the bending angle, the number of bendings, and the like. When the number of bendings is large, it is especially difficult to accurately estimate the amount of spring-back. Thus, such a method is insufficient as a countermeasure for spring-back.
Patent Document 1 discloses a countermeasure in which a metal tube with spring-back due to bending is corrected by pressing and forming a number of annular grooves (concave grooves) on an outer peripheral surface of the metal tube at predetermined pitches. According to the countermeasure, since the metal tube is uniformly deformed by plastic deformation, the spring-back is eliminated and a desired shape precision can be secured.
According to the countermeasure for spring-back described in Patent Document 1, forming the annular grooves increases the section stiffness. However, as a number of annular grooves are provided on the metal tube in the lengthwise direction, there is a problem in which overall bending stiffness decreases. In addition, forming recesses of the press die have a semi-circular profile that reflects the cross-sectional shape of the metal tube. Therefore, it is difficult to stably set the metal tube that is deformed due to the spring-back, and thus, a forming defect is likely to occur.
Moreover, there is another problem in which forming annular grooves causes retraction of material at the tube ends, thereby significantly deteriorating end-face precision.    Patent Document 1: Japanese Patent Application Publication No. JP-A-2005-81393